This disclosure is directed to a system and method for controlling an optoelectronic oscillator (“OEO”). In a preferred embodiment, the OEO is capable of producing repetitive electronic sine or any arbitrary wave and/or electrically modulated continuous wave or pulsed optical signals.
OEOs are an optical and electronic hybrid device and use optical modulation to produce electrical output oscillations in RF and microwave frequencies that can exhibit narrow spectral line widths and ultra low phase noise in comparison with the conventional RF and microwave signal sources. Optical resonators intrinsically can support variation of resonant modes of light called whispery gallery modes (WGMs), hence many oscillation modes can be sustained in an OEO so long the gain of active regenerative feedback loop compensates and exceeds the loop looses.
Generally, optoelectronic oscillators receive pump continuous energy from an optical source, such as a laser, in addition to energy in the form of direct current (DC) power from a power supply. The energy is converted into radio frequency (RF) and microwave signals based on a filtering mechanism. Optoelectronic oscillators typically experience a low loss, low temperature sensitivity, and can be achieved at a relatively small size, as compared to conventional electronic delay elements of similar delay times. These benefits often lead to a high quality factor and greater stability for the optoelectronic oscillators in both the short term and the long term, as compared to electronic oscillators.
An optoelectronic oscillator generally utilizes a modulator, such as an optical modulator, to convert continuous wave light energy from the laser into a modulated stable, spectrally pure optical signal (e.g., RF signal, microwave signal). Energy from the laser passes through the modulator and then fed back to itself. First, the optical signal passes through a delay line (e.g., a fiber optic cable). The optical signal traveling through the delay line is then detected by a photodetector and converted to an electrical signal. The electrical output of the photodetector is amplified, filtered, and fed back to the modulator in a closed loop. This configuration supports self-sustained oscillations, provided that the electrical feedback signal fed to the modulator meets certain oscillation conditions in terms of its amplitude and phase. The frequency of the optoelectronic oscillator's output is controlled by several factors, such as, the fiber delay length, the operating condition of the modulator, and the band pass characteristics of the filter used to filter the oscillating signal.
An important aspect of sustaining a pure sinusoidal oscillating signal in any oscillator is filtering the sustaining signal from the surrounding sources that contribute to close-in to carrier phase noise. Phase noise reduction may be accomplished in one of several ways.
Some optoelectronic oscillators reduce phase noise by forcing oscillations using injection-locking (IL). In an injection-locked oscillator, a stable master oscillator pulls a less stable slave oscillator to a harmonic frequency of the master oscillator, within a range of detuning frequencies known as the frequency locking range. Pulling the frequency of the slave oscillator to that of the master oscillator reduces the slave oscillator's frequency variations within the frequency locking range, thereby also reducing phase noise of the slave oscillator within the frequency locking range.
Other optoelectronic oscillators reduce phase noise using a phase locked loop (PLL). In a PLL oscillator, the phase of a reference signal (e.g., a master signal from an OEO) is compared to the phase of the oscillator's signal using a phase comparator. The difference between the phase of each the reference signal and the oscillator is used to generate a phase error output, which is a variable signal used to correct deviations in the phase and/or frequency of the slave oscillator.
While the currently known advances in phase noise reduction are effective at raising the quality factor and increasing stability for oscillators, these advances alone do not provide a stable enough signal to satisfy some oscillator applications in both current and future technology. For example, cellular systems (e.g., broadband MIMO, UWB, 4G LTE, etc.) rely on fitting an ever increasing amount of data into a limited bandwidth that gets even more crowded over time. In order to fit this data into such bandwidth, it is preferable that the frequency at which the data is transmitted is kept to as narrow a bandwidth as possible. Additionally, it is preferable that the frequency at which such data is transmitted is locked as precisely as possible, and that unwanted shifts in the frequency and phase are kept to a minimum.
Each of IL and PLL noise reduction does not achieve a sufficiently stable signal to accommodate the continuously increasing demand to fit more and more data over a limited bandwidth. As a result, there is a need for an optoelectronic oscillator having a further improved quality factor, further reduced phase noise, and (as a result) increased stability.